The del_gendisk() function uninitializes the disk-specific data
structures, including the bdi structure, without telling anyone
else.  Once this happens, any attempt to call mark_buffer_dirty()
(for example, by ext4_commit_super), will cause a kernel OOPS.

Fix this for now until we can fix things in an architecturally correct
way.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>